Preheat the oven to 180°C/350°F/gas 4. Line a 24cm square baking tin with greaseproof paper.
Snap the chocolate into a large bowl, add the butter and place over a pan of simmering water, until melted.
Stir through the nuts.
Sift the cocoa powder and flour into a separate bowl, add the baking powder and sugar, then mix together.
Add the dry ingredients to the chocolate and nut mixture and stir together well. Beat the eggs, then mix in until you have a silky consistency.
Pour the brownie mix into the baking tin, and place in the oven for around 25 minutes.
Serve warm with ice cream and enjoy!